About three feet long and glistening black with a pink, fleshy appendage, it looks like the entrails of an alien from a bad horror film. In fact, it is a giant shipworm.

Discovered in the mud of a shallow lagoon in the Philippines, a living creature of the species has never been described before – even though its existence has been known for more than 200 years thanks to fossils of the baseball bat-sized tubes that encase the creature.

“Although people have known [these animals] exist, they didn’t know the simplest things about them,” said Dan Distel of Northeastern University’s marine science centre and co-author of the study published in the journal PNAS. “It was a very mysterious organism.”

Distel points out that a description based on a museum specimen was made decades ago, but adds that the creature was not well preserved. “We think, among living biologists, anyway, our group are probably the only group that has seen living specimens,” he said.

With the Linnaean classification Kuphus polythalamia, the creature lives in the mud inside a long tube made of calcium carbonate secreted by the animal. The tube forms a casing for the beast, including its head. “If they want to grow, they have to open that end of that tube, so somehow dissolve or reabsorb that cap on the bottom, grow, extend the tube down further into the mud, and then they seal it off again,” said Distel.

The end of the tube, adds Distel, is Y-shaped and surrounds two siphons – water is drawn in through one, pushed through the creature’s gills, then expelled through the other.

Despite being known as a shipworm – a nod to its relatives’ diet of submerged wood – the animal is actually a type of clam. It has a modified version of two clam shells at its head, while the body stretches out behind. “Its body has been stretched out through evolution so that it no longer fits between the two shells,” said Distel.

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The team stumbled across a clue to the creatures’ whereabouts thanks to a YouTube video of a Philippine television news report. They asked academics in the region about possible locations and subsequently located a crop of the tubes in a lagoon replete with rotting wood. The location, adds Distel, remains a secret, to prevent the site being disturbed by shell collectors.

Divers collected tubes found sticking upwards around 10ft below the surface. “That tube is anywhere from maybe 75%-80% buried in the mud,” said Distel. About half a dozen were shipped to the laboratory, where the team tentatively opened one.

“It was really quite amazing,’ said Distel. “I didn’t even have any idea how to open it, but I thought: ‘Carefully.’”

The appearance of the shipworm when it slid out of the tube came as a surprise to the researchers. “That colour of the animal is sort of shocking,” Distel said. “Most bivalves are greyish, tan, pink, brown, light beige colours. This thing just has this gunmetal-black colour. It is much beefier, more muscular than any other bivalve I had ever seen.”

But it isn’t just its discovery that stunned researchers: the giant shipworm is also surprising for its mode of survival. “Gigantism is usually an indication of ample nutrients,” said Distel. Other shipworms feed on submerged wood with the aid of wood-degrading bacteria that live in their gills, but the newly discovered specimen had only a tiny digestive system, while the fact that the creature was enclosed in a tube suggested it was not eating mud.

Further work revealed that the creature relies on bacteria in its gills that use hydrogen sulphide in the water as an energy source. That energy is then used to turn carbon dioxide into nutrients for the shipworm.

The discovery, Distel adds, sheds light on the evolution of symbiotic relationships between sulphur-oxidising organisms and other creatures, and backs up the possibility that sunken wood might have played a role in how such species ended up in locations such as deep sea hydrothermal vents. “To me it was almost like finding a dinosaur – something that was pretty much only known by fossils,” he said.

Simon Watt, biologist, TV presenter and president of the Ugly Animal Preservation Society, welcomed the discovery of the giant shipworm. “It might well be monstrous, but that does not mean that it isn’t marvellous,” he said, pointing out that the creature has evolved to live in an environment that is also “pretty disgusting”. “If you are down living among murky dirt, then aesthetics are surely not your number one priority,” he added.